I'd suggest that you wiggle that monitor cable where it enters the monitor and where it is connected to the computer. Also look at the connector pins. If that fails you might try borrowing another from someone else.
There are contacts in the cable that connect the monitor to the video of your computer that communicate digitally EDID information. That information tells the operating system what constraints the monitor has on how fast it can refresh horizontally and vertically, how fast bit data can be clocked to it, and so on. It also suggests a good resolution it can handle well. It is sent in several blocks.
If a pin has been bent in a connector, or a wire has busted that information may not make it and the video drivers will only do very conservative resolutions.
You may find additional information in /var/log/Xorg.0.log looking for the string "EDID".